Mixing devices and containers, in particular fermenters as described supra have been known in the art for quite a while and are used in particular in the field of biogas systems that are used for generating renewable energy.
Mixing devices as recited supra are subject to a certain amount of normal wear during operations. The wear occurs primarily in the bearings and in the transmission of the mixing devices. Depending on the quality of the mixing device, a failure occurs sooner or later due to a failure of at least one of the wear parts, unless a preventive replacement of wear parts is already performed prior to that. In case of a failure the wear parts have to be replaced in order to be able to operate the mixing device again. In fermenters with plural mixing devices, a temporary failure of an individual mixing device may be tolerable for a certain time period. The reduced circulation of the substrate arranged in the container, however, retards the desirable biological break-down process which eventually leads to a reduction of the through-put and energy output of the biogas plant. Such losses significantly degrade the economics of the biogas system. A longer failure of a mixing device or even of plural mixing devices is therefore not acceptable.
Therefore, the failed mixing devices are repaired or replaced. For this purpose it is frequently required to open the fermenter, empty it and stop operations of the biogas plant. Shut-down times caused thereby induce significant cost which should rather be avoided.
A solution of this problem is taught for example by EP 2 064 308 A1 which describes a mixing device which can be changed during ongoing operations of the fermenter. This is achieved in that a lower end of the shaft of the mixing device which is oriented towards a base plate of the container and which end together with a rotation bearing is configured as a bearing cartridge inserted into the hollow shaft forms the bearing device wherein the lower end of the shaft is loosely inserted into a receiving device of the container. This receiving device is configured to block a rotation of the support device of the mixing device wherein the support device is configured as a rolling element bearing in order to support the mixing device. However, the mixing device can be easily lifted out of the receiving device without having to take particular measures. In particular, it is not required to disengage the mixing device at its lower end from the receiving device through tools or similar.
The known configuration, however, has disadvantages in that it requires on the one hand a mandatory vertical operating position of the mixing device shaft and on the other hand requires a fermenter whose ceiling device is formed by a rigid concrete ceiling. This rigid concrete ceiling is used as a framework for receiving the mixing devices which are lowered from the top side into the fermenter. A ceiling configuration of this type is disadvantageous in that it requires on the one hand side a high level of planning and has to be computed and planned for each container of each project separately, and on the other hand delays the construction of the fermenter since a ceiling structure of this type is rather difficult to produce.
From this point of view, fermenters of this type with membrane closures are very advantageous. Membranes of this type are connected gas-tight to circumferential walls of the fermenter and then bulge outward due to biogas forming in the interior cavity of the fermenter and form so-to-speak a tent roof over the fermenter. Constructing a ceiling device of this type in the form of a flexible membrane can, therefore, be performed in a much quicker and cost-effective manner. Furthermore, most of the planning complexity recited supra can be omitted. However, using vertically oriented mixing devices is not possible in plants of this type since supporting the mixing device at its upper end cannot be performed easily without a frame structure. Namely, the membrane itself is not configured to react incurred support forces.
Therefore, fermenters that are enclosed with membranes of this type are typically operated with mixing devices which are configured for circulating the substrate wherein the mixing devices are laterally inserted into the fermenter through the wall. For this purpose slanted mixing devices and also so-called rod mixing devices can be used. Slanted mixing devices are known for example from the documents DE 20 2004 004 101 U1 and WO 2007/110775 A2. These illustrate mixing devices which are laterally run through the walls of the respective fermenter out of the inner cavity in outward direction and which are supported in or at the walls. The illustrated systems, however, have the disadvantage that the fermenter can only be filled up to a relatively low filling level which terminates in any case below the lateral mounting opening in the wall of the container. The height of the fermenter can, therefore, only be used partially. Furthermore, there is a problem in that due to the configuration inherent closeness of the shaft of the mixing device to the wall of the container in the upper end section of the mixing device, the associated mixing elements are arranged very close to the walls. The associated mixing elements, however, have to be arranged at least far enough from the wall so that they do not impact the wall when the shaft of the mixing device rotates.
Furthermore, a heatable mixing device for fermentation containers is known from DE 10 2004 027 077 A1, wherein the mixing device is provided for introducing mechanical and also thermal-energy into a fermentation container. The known mixing device includes a heatable mixing shaft and heat-conducting surfaces attached to the mixing shaft wherein mixing blades are arranged at the mixing shaft. In installed position, the shaft axis is inclined relative to vertical by an angle of approximately 45°. The shaft support is arranged in a bearing base (so-called mounting block) fixated at the container. The inclination angle of the lower support device fixated at the container is adjustable during assembly when the fermentation container is empty. After the bearing is worn out, the container has to be emptied in order to be able to replace the bearing device.
The same problem of mandatory container or reactor emptying occurs in the mixing device according to DE 10 2007 034 463 A 1. A lower element of the bearing device is fixated at the container base so that a replacement is not possible when the reactor is full. Furthermore, the mixing device is vertically oriented in installed position.
DE 82 12 809 U1 discloses a mixing device in which a base mounted support device is provided. It is an object of the invention described in this document to facilitate a flow direction in the substrate to be mixed which flow direction is parallel to the mixing device shaft. When the base mounted bearing device is damaged, the container has to be emptied in order to perform a replacement of the bearing device.
DE 20 2006 011 144 U1 teaches a mixing device in a slanted installed position with a bearing device that is fixated at the container. This causes the problems recited supra when a bearing is damaged or when a routine bearing change needs to be performed.
DE 20 2009 010 167 U1 teaches a mixing in a so-called submerged motor configuration wherein a mixing device mast is rotatably supported on a pinion at a base and configured to be rotated by a first crank. The mixing device which is moveably supported in longitudinal direction of the mast at a sleeve can be optionally removed out of the fermentation mass through a pull cable or lowered into the fermentation mass. A slanting of the mixing device in its operating position is neither provided nor helpful. Also EP 2 636 444 A1 discloses a submerged motor mixing device of this type.
Last not least WO 2011/139209 A1 discloses a suspended mixing device whose lower end is suspended at the container base with a spring-loaded chain. On the one hand it is necessary to empty the container when a bearing at the bottom side of the mixing device shaft is worn, and on the other hand the suspended support for receiving the tension forces impacting the mixing device requires a high load-bearing configuration for the upper mixing device attachment which can typically only be configured as a concrete ceiling for the fermentation tank. A slanted arrangement of the mixing device is not provided.